People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

ADHD Is Just as Prevalent in the Elderly as in Children

While Attention Deficit Hyperactivity Disorder is a condition that is usually associated with the young, experts claim that the elderly can also suffer from ADHD.

A new study revealed that the hyperactivity disorder is almost as common in people over the age of 60 as in children.

The latest findings by Dutch researchers go against the popular perception that many children grow out of the condition.

It is estimated that between 3 percent and 7 percent of school-aged children suffer from ADHD, a condition which causes a short attention span, restlessness and fidgeting.

It is believed that about a third will grow out of the condition before they become teenagers, and many of the others who still have ADHD as teenagers will see their symptoms improve gradually with age even if they are not completely cured.

Researchers said that it is not known exactly how prevalent ADHD is among adults, particularly among older adults.

Researchers from the University Medical Centre in Amsterdam assessed 231 Dutch adults between the age of 60 and 94 for the study.

Participants were all asked to complete surveys that were used to diagnose the condition. Some of the questions in the survey included whether participants found it difficult to watch a film from the beginning to end or if they were easily distracted by their own thoughts.

Researchers found that while 2.8 percent of the participants in the study had the condition, the prevalence of ADHD appeared to diminish with age.

While about 4 percent of adults between the age of 60 and 70 were found to have ADHD, only 2.1 percent of those between the age of 70 and 94 have the condition.

Researchers suggest that this may be because symptoms of the condition improve with age or it may be because their diagnostic test does not effectively detect ADHD in older adults.

"Little is known about ADHD in old age and this is the first epidemiological study on ADHD in older people," Lead researcher Marieke Michielsen said in a press release.

"With a prevalence of 2.8 per cent, our study demonstrates that ADHD does not face or disappear with age, and that it is a topic very much worthy of further study," Michielsen said.

ADHD was not recognized as a medical condition until the 1980s. It is the most common childhood psychiatric condition, and affects about 4.4 percent of adults.

ADHD is a treatable neuropsychiatric disorder characterized by inattention, excessive physical activity and impulsivity in children. Most adults with the condition exhibit problems with inattention, which manifests as disorganization, forgetfulness, unreliability, and difficulty in planning, task completion, task shifting and time management.

Researchers said that because ADHD was only fairly recently recognized as a condition, it was likely that many older adults who have the disorder have never been properly diagnosed.